Voice over Wifi – Which RAN to Prefer?

There are a number of devices and network operators out in the wild now which do not only support Voice over LTE (VoLTE) but also Voice over Wifi (VoWifi) and can even hand-over ongoing calls between the two radio access networks. But there is one thing: Which RAN should be preferred if both LTE and Wifi are available? The answer is not as simple as it looks at first.

VoWifi – Cellular Preferred

Let’s say the answer is that in case both cellular and Wifi is available, the cellular network is preferred over Wifi for making and receiving calls. This is what’s called VoWifi ‘cellular preferred’. It’s a good choice in case the Wifi network at home or at the office is often overloaded or has a very limited uplink capacity. A cat video sent by someone else while a call is ongoing and clogging the uplink does not matter as the voice call uses the cellular network. VoWifi ‘cellular preferred’ has its downsides, too, however. If LTE network coverage is weak and one moves further indoor, a handover from LTE to Wifi of an ongoing call is required. If the network is quicker than the device, however, the ongoing VoLTE calls is handed over to GSM. From there, no handover to Wifi is possible and the call will drop if the user runs out of GSM coverage. But there is a least a fair chance the mobile is faster and the handover to Wifi initiated by the mobile device happens before the network initiated handover to GSM. If there is only GSM and Wifi coverage, a handover won’t work at all. Also, when GSM reception levels are very low it can happen that the paging for an incoming call is received but the transmission power in the uplink direction is not strong enough to reach the base station. If the call had only come in over Wifi in that case…

VoWifi – Wifi Preferred

O.k., so let’s say the answer is that in case cellular and Wifi is present, Wifi is preferred for making and receiving voice calls. This solves the handover issues described above so that’s a nice thing. Unfortunately the main problem of such a setting is that quality of service is not ensured on the Wifi network. Especially in hotel networks or when somebody at home clogs the uplink with a massive upload, sound quality quickly deteriorates significantly. This is not a theoretical issue, it happens. Another problem of this setting is that in case only GSM and Wifi is available, a call will drop when the user leaves the Wifi coverage area, e.g. when he is leaving the house. Also not nice.

What to Prefer When Roaming?

Yet another case is when the user is in another country and his home network operator allows him to use Wifi calling abroad. Calls made over the cellular network abroad are usually priced differently than calls made at home. As Voice over Wifi is location independent and the IMS network can’t really tell whether the user is in his home country or not, VoWifi calls are always billed as calls from the home country. In short, cellular calls while roaming are billed differently than VoWifi calls abroad. And some cellular calls might be cheaper than Wifi calls abroad. An example are local calls in the roaming country which are sometimes cheaper when done over cellular then over Wifi in which they would be billed as having originated from the home country.

Prefer Some Wifi Networks But Not Others

One other thing that requires some thought is that not all Wifi networks are good Wifi networks. While one might want to, as an example, use VoWifi at home and at work, one wouldn’t want to use VoWifi in a hotel in the home country but perhaps in a hotel at the other end of the world to escape prohibitive roaming charges.

Summary and Potential Solutions

From my point of view the discussion above shows quite clearly that there is no simple ‘one size fits’ all solution to VoWifi and RAN preference. As someone who likes to have choice, having the option to switch between cellular preferred and Wifi preferred depending on my circumstances would be the ideal solution. But I guess most people wouldn’t know how to set things in the first place nor would they want to care. So a solution might be to put more ‘bearer intelligence’ into the device such as:

Prefer cellular LTE but not GSM and don’t stay in the cellular network if it is very weak and there is good Wifi available.

When on Wifi and there are outages or long delays on the speech path (which might be very difficult or impossible to detect in the transmission direction) change to LTE with an ongoing call.

Learn if incoming calls on GSM or LTE fail because the paging can be received but the network connection attempt fails. If it happens at the same location, e.g. always where the same Wifi is available, leave GSM or LTE earlier and connect to VoWifi even in cellular preferred mode.

When LTE and Wifi is available, prefer the LTE network but perhaps the device should not tell the network that it supports GSM and UMTS so handovers to circuit switched networks are prevented.

Obviously not all ideas go hand in hand with each other and are even contradictory. In the end, even if a device is made super smart, it will still come down to probabilities…